Commit 4969c1192d15afa3389e7ae3302096ff684ba655 "mm: fix swapin race condition"
is now agreed to be incomplete. There's a race, not very much less likely
than the original race envisaged, in which it is further necessary to check
that the swapcache page's swap has not changed.
Here's the reasoning: cast in terms of reuse_swap_page(), but probably could
be reformulated to rely on try_to_free_swap() instead, or on swapoff+swapon.
A, faults into do_swap_page(): does page1 = lookup_swap_cache(swap1)
and comes through the lock_page(page1).
B, a racing thread of the same process, faults on the same address:
does page1 = lookup_swap_cache(swap1) and now waits in lock_page(page1),
but for whatever reason is unlucky not to get the lock any time soon.
A carries on through do_swap_page(), a write fault, but cannot reuse
the swap page1 (another reference to swap1). Unlocks the page1 (but B
doesn't get it yet), does COW in do_wp_page(), page2 now in that pte.
C, perhaps the parent of A+B, comes in and write faults the same swap
page1 into its mm, reuse_swap_page() succeeds this time, swap1 is freed.
kswapd comes in after some time (B still unlucky) and swaps out some
pages from A+B and C: it allocates the original swap1 to page2 in A+B,
and some other swap2 to the original page1 now in C. But does not
immediately free page1 (actually it couldn't: B holds a reference),
leaving it in swap cache for now.
B at last gets the lock on page1, hooray! Is PageSwapCache(page1)?
Yes. Is pte_same(*page_table, orig_pte)? Yes, because page2 has
now been given the swap1 which page1 used to have. So B proceeds
to insert page1 into A+B's page_table, though its content now
belongs to C, quite different from what A wrote there.
B ought to have checked that page1's swap was still swap1.
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: stable@xxxxxxxxxx
---
mm/memory.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)
--- 2.6.36-rc4/mm/memory.c 2010-09-12 17:34:03.000000000 -0700
+++ linux/mm/memory.c 2010-09-19 18:23:43.000000000 -0700
@@ -2680,10 +2680,12 @@ static int do_swap_page(struct mm_struct
delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
/*
- * Make sure try_to_free_swap didn't release the swapcache
- * from under us. The page pin isn't enough to prevent that.
+ * Make sure try_to_free_swap or reuse_swap_page or swapoff did not
+ * release the swapcache from under us. The page pin, and pte_same
+ * test below, are not enough to exclude that. Even if it is still
+ * swapcache, we need to check that the page's swap has not changed.
*/
- if (unlikely(!PageSwapCache(page)))
+ if (unlikely(!PageSwapCache(page) || page_private(page) != entry.val))
goto out_page;
if (ksm_might_need_to_copy(page, vma, address)) {
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxxx For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>